It’s risky behavior: that much is clear. If such actions were isolated specifically to a single group of students, that might be one thing, but if there’s anyone believing that all of these students would immediately self-isolate for the next 2 weeks after contact, then they should think again. They could easily endanger others... so yes: while I believe in herd immunity (I remember chicken-pox parties), I don’t believe that having idiot college kids try to self-administer that is a good idea.
I don’t think they planned on self isolating - they knowingly had people at the parties who were positive. I think it would be beneficial to the vulnerable if these students recruited even more partiers and they tested positive too. It is far less lethal than the flu for them. As for your concern for the vulnerable getting exposed by them - they need to take their own precautions. I believe in liberty over collective health.